Johnstone Family Professor, Department of Psychology; Harvard University; Author, The Sense of Style

Have Humans Stopped Evolving?

Ten years ago, I wrote:

For ninety-nine percent of human existence, people lived as foragers in small nomadic bands. Our brains are adapted to that long-vanished way of life, not to brand-new agricultural and industrial civilizations. They are not wired to cope with anonymous crowds, schooling, written language, government, police, courts, armies, modern medicine, formal social institutions, high technology, and other newcomers to the human experience.

And:

Are we still evolving? Biologically, probably not much. Evolution has no momentum, so we will not turn into the creepy bloat-heads of science fiction. The modern human condition is not conducive to real evolution either. We infest the whole habitable and not-so-habitable earth, migrate at will, and zigzag from lifestyle to lifestyle. This makes us a nebulous, moving target for natural selection. If the species is evolving at all, it is happening too slowly and unpredictably for us to know the direction. (How the Mind Works)

Though I stand by a lot of those statements, I've had to question the overall assumption that human evolution pretty much stopped by the time of the agricultural revolution. When I wrote these passages, completion of the Human Genome Project was several years away, and so was the use of statistical techniques that test for signs of selection in the genome. Some of these searches for "Darwin's Fingerprint," as the technique has been called, have confirmed predictions I had made. For example, the modern version gene associated with language and speech has been under selection for several hundred thousand years, and has even been extracted from a Neanderthal bone, consistent with my hypothesis (with Paul Bloom) that language is a product of gradual natural selection. But the assumption of no-recent-human-evolution has not.

New results from the labs of Jonathan Pritchard, Robert Moyzis, Pardis Sabeti, and others have suggested that thousands of genes, perhaps as much as ten percent of the human genome, have been under strong recent selection, and the selection may even have accelerated during the past several thousand years. The numbers are comparable to those for maize, which has been artificially selected beyond recognition during the past few millennia.

If these results hold up, and apply to psychologically relevant brain function (as opposed to disease resistance, skin color, and digestion, which we already know have evolved in recent millennia), then the field of evolutionary psychology might have to reconsider the simplifying assumption that biological evolution was pretty much over and done with 10-000 — 50,000 years ago.

And if so, the result could be evolutionary psychology on steroids. Humans might have evolutionary adaptations not just to the conditions that prevailed for hundreds of thousands of years, but also to some of the conditions that have prevailed only for millennia or even centuries. Currently, evolutionary psychology assumes that any adaptation to post-agricultural ways of life are 100% cultural.

Though I suspect some revisions will be called for, I doubt they will be radical, for two reasons. One is that many aspects of the human (and ape) environments have been constant for a much longer time than the period in which selection has recently been claimed to operate. Examples include dangerous animals and insects, toxins and pathogens in spoiled food and other animal products, dependent children, sexual dimorphism, risks of cuckoldry and desertion, parent-offspring conflict, risk of cheaters in cooperation, fitness variation among potential mates, causal laws governing solid bodies, presence of conspecifics with minds, and many others. Recent adaptations would have to be an icing on this cake -- quantitative variations within complex emotional and cognitive systems.

The other is the empirical fact that human races and ethnic groups are psychologically highly similar, if not identical. People everywhere use language, get jealous, are selective in choosing mates, find their children cute, are afraid of heights and the dark, experience anger and disgust, learn names for local species, and so on. If you adopt children from a technologically undeveloped part of the world, they will fit in to modern society just fine. To the extent that this is true, there can't have been a whole lot of uneven psychological evolution postdating the split among the races 50-100,000 years ago (though there could have been parallel evolution in all the branches).